The adsorption of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) on polystyrene latex particles of low negative charge has been examined as a function of surfactant concentration by estimation of the hydrodynamic radius by dynamic light scattering measurements. Adsorption isotherms were also determined by a surface tension technique. The "hairy" layer at the latex surface is considered to be an important factor determining surfactant adsorption. The DTA(+) ions are initially firmly bound at the extremities of the polystyrene chains through both charge attraction between the quaternary ammonium head groups of DTAB and the terminal sulfate entities of the polymer chain and by hydrophobic interaction between the alkyl tails and the polymer backbone. This leads to L-type isotherms in the salt-free system and at 1 mM added NaBr. Further adsorption of surfactant subsequently occurs around the initially adsorbed DTA(+) ions through hydrophobic interaction between the alkyl tails of the surfactant, finally leading to coiling of the polymer segments and formation of semiglobular aggregates at the latex surface. The clear two-step character of the isotherms and the corresponding contraction of the adsorbed layer reflect this adsorption mechanism. At high ionic strength, enhanced adsorption leads to close packing of the DTA(+) ions because of effective screening of the interactions between the quaternary ammonium head groups. Thus semiglobular aggregates of greater area can form at the latex surface. Further adsorption above the cmc will eventually cause coalescence of adjacent aggregates, forming an adsorbed monolayer comprised of a continuous hydrophobic phase intercalated by the heads of surfactants and the surface. Effective screening of interactions between the head groups of the adsorbed DTA(+) ions as well as enhanced hydrophobic interaction between the alkyl tails of the surfactants results in an S-type isotherm at high ionic strength. The adsorbed amount decreases with increasing alkyl chain length of the cationic surfactant and this effect is considered to derive from the effect of the "hairy" layer at the latex surface. Enhanced hydrophobic interaction promotes bridging of adjacent polymer chains on adsorption of the cationic surfactant leading to aggregation of chains and decreased adsorption.